PeriodicReloadingTrigger

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 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.commons.configuration2.reloading;

import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ScheduledFuture;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.TimeUnit;

import org.apache.commons.lang3.concurrent.BasicThreadFactory;

/**
 * <p>
 * A timer-based trigger for reloading checks.
 * </p>
 * <p>
 * An instance of this class is constructed with a reference to a {@link ReloadingController} and a period. After
 * calling the {@code start()} method a periodic task is started which calls
 * {@link ReloadingController#checkForReloading(Object)} on the associated reloading controller. This way changes on a
 * configuration source can be detected without client code having to poll actively. The {@code ReloadingController}
 * will perform its checks and generates events if it detects the need for a reloading operation.
 * </p>
 * <p>
 * Triggering of the controller can be disabled by calling the {@code stop()} method and later be resumed by calling
 * {@code start()} again. When the trigger is no more needed its {@code shutdown()} method should be called.
 * </p>
 * <p>
 * When creating an instance a {@code ScheduledExecutorService} can be provided which is then used by the object.
 * Otherwise, a default executor service is created and used. When shutting down this object it can be specified whether
 * the {@code ScheduledExecutorService} should be shut down, too.
 * </p>
 *
 * @since 2.0
 * @see ReloadingController
 */
public class PeriodicReloadingTrigger {
    /** The executor service used by this trigger. */
    private final ScheduledExecutorService executorService;

    /** The associated reloading controller. */
    private final ReloadingController controller;

    /** The parameter to be passed to the controller. */
    private final Object controllerParam;

    /** The period. */
    private final long period;

    /** The time unit. */
    private final TimeUnit timeUnit;

    /** Stores the future object for the current trigger task. */
    private ScheduledFuture<?> triggerTask;

    /**
     * Creates a new instance of {@code PeriodicReloadingTrigger} and sets all parameters.
     *
     * @param ctrl the {@code ReloadingController} (must not be <b>null</b>)
     * @param ctrlParam the optional parameter to be passed to the controller when doing reloading checks
     * @param triggerPeriod the period in which the controller is triggered
     * @param unit the time unit for the period
     * @param exec the executor service to use (can be <b>null</b>, then a default executor service is created
     * @throws IllegalArgumentException if a required argument is missing
     */
    public PeriodicReloadingTrigger(final ReloadingController ctrl, final Object ctrlParam, final long triggerPeriod, final TimeUnit unit,
        final ScheduledExecutorService exec) {
        if (ctrl == null) {
            throw new IllegalArgumentException("ReloadingController must not be null!");
        }

        controller = ctrl;
        controllerParam = ctrlParam;
        period = triggerPeriod;
        timeUnit = unit;
        executorService = exec != null ? exec : createDefaultExecutorService();
    }

    /**
     * Creates a new instance of {@code PeriodicReloadingTrigger} with a default executor service.
     *
     * @param ctrl the {@code ReloadingController} (must not be <b>null</b>)
     * @param ctrlParam the optional parameter to be passed to the controller when doing reloading checks
     * @param triggerPeriod the period in which the controller is triggered
     * @param unit the time unit for the period
     * @throws IllegalArgumentException if a required argument is missing
     */
    public PeriodicReloadingTrigger(final ReloadingController ctrl, final Object ctrlParam, final long triggerPeriod, final TimeUnit unit) {
        this(ctrl, ctrlParam, triggerPeriod, unit, null);
    }

    /**
     * Starts this trigger. The associated {@code ReloadingController} will be triggered according to the specified period.
     * The first triggering happens after a period. If this trigger is already started, this invocation has no effect.
     */
    public synchronized void start() {
        if (!isRunning()) {
            triggerTask = getExecutorService().scheduleAtFixedRate(createTriggerTaskCommand(), period, period, timeUnit);
        }
    }

    /**
     * Stops this trigger. The associated {@code ReloadingController} is no more triggered. If this trigger is already
     * stopped, this invocation has no effect.
     */
    public synchronized void stop() {
        if (isRunning()) {
            triggerTask.cancel(false);
            triggerTask = null;
        }
    }

    /**
     * Returns a flag whether this trigger is currently active.
     *
     * @return a flag whether this trigger is running
     */
    public synchronized boolean isRunning() {
        return triggerTask != null;
    }

    /**
     * Shuts down this trigger and optionally shuts down the {@code ScheduledExecutorService} used by this object. This
     * method should be called if this trigger is no more needed. It ensures that the trigger is stopped. If the parameter
     * is <b>true</b>, the executor service is also shut down. This should be done if this trigger is the only user of this
     * executor service.
     *
     * @param shutdownExecutor a flag whether the associated {@code ScheduledExecutorService} is to be shut down
     */
    public void shutdown(final boolean shutdownExecutor) {
        stop();
        if (shutdownExecutor) {
            getExecutorService().shutdown();
        }
    }

    /**
     * Shuts down this trigger and its {@code ScheduledExecutorService}. This is a shortcut for {@code shutdown(true)}.
     *
     * @see #shutdown(boolean)
     */
    public void shutdown() {
        shutdown(true);
    }

    /**
     * Gets the {@code ScheduledExecutorService} used by this object.
     *
     * @return the associated {@code ScheduledExecutorService}
     */
    ScheduledExecutorService getExecutorService() {
        return executorService;
    }

    /**
     * Creates the task which triggers the reloading controller.
     *
     * @return the newly created trigger task
     */
    private Runnable createTriggerTaskCommand() {
        return () -> controller.checkForReloading(controllerParam);
    }

    /**
     * Creates a default executor service. This method is called if no executor has been passed to the constructor.
     *
     * @return the default executor service
     */
    private static ScheduledExecutorService createDefaultExecutorService() {
        final ThreadFactory factory = new BasicThreadFactory.Builder().namingPattern("ReloadingTrigger-%s").daemon(true).build();
        return Executors.newScheduledThreadPool(1, factory);
    }
}